Fixed Condensers

Radio Record, Volume V, Issue 27, 15 January 1932, Page 12

Fixed Condensers

Test and Working Voltages This week we deal with. the third question in Section B of. the Radio Serviceman’s Examination. The question reads: "A fixed condenser is marked tested 500 volts d.c. State the highest voltages you would apply to it for continuous working (a) in a battery-operated amplifier; (b) if coupled across an a.c. circuit. Explain how you would arrive at these values."

HE question does not call for a treatise on condensers. It asks for a simple fact which every serviceman should know. Condensers are tested, unless otherwise stated, by the flash method, which means that a high potential is flashed across the condenser. Some of the better known makes of condensers are tested for a few seconds at this high voltage, but this is not the potential at which the condenser is designed to operate and for normal purposes; one must take half the test voltage as the working voltage. Thus, in this case, one would not apply more than 250 volts to the condenser for safe working. Much depends upon the position the condenser will occupy in the amplifier. If used in an output filter there is a chance that higher yoltages will be built up through the loudspeaker circuit. If a power valve empioying 250 volts on the plate were used in the last stage, it would be unwise to associate with the output filter a condenser tested at 500 volts. It would he better to use a higher test condenser or limit the voltage to 200.

B. In an a.c. circuit conditions are somewhat different. The voltage shown on an a.c. meter is not the total voltage across the circuit. This voltage shown is known as the r.m.s., and is the average one, but owing to the nature of alternating current there is a peak voitage, Which is 1.414 times the r.m.s. voltage, and this we must allow for in designing the circuit. If the safe working margin of the condenser is 250 the peak value of the current must not exceed this figure. If we applied mathe matics to the problem it would be found that y.m.s, yalue is equal to .707 of the peak value and .707 of 250 is 175 approximately. Thus for continuous working in an a.c, amplifier no higher voltage than this should be used, Where surges have to be allowed for, and they are not infrequent in a.c, operated amplifiers, a still lower value of working voltage must be taken. Surges are rarely more than 10 per cent. working voltages, and this brings the "safe" voltage down to abont 150. It is unlikely, however, that this point was implied in the question.

25 with a shortwave’ detcetor haying no audio stages following. What is the size of the condenser in my circuit? ook wi 8 A.: Approximately. .0001.-.00025. .2, Would I make the secondaries in accordance. with the table in the "R.R." and experiment with these?-Yes. 3. Would the method of supply the ht. to the 8.G. and"plate be, in your estimation, satisfactory. A.: Yes; we think the method would be quite satisfactory,-although it may be better to make-the. screen voltage variable and the. plate yoltage fixed. J IE.H. (Marborough).~I have a bat- * tery set earthed to the water service to which also is earthed the telephone. When the telephone rings it can be heard in the speaker. Will this injure the set? A.: No; but if you want to avoid the annoyance have a separate earth for the set-say a water pipe driven into the ground. \ BM (Rongotea).-Of the aeriah you suggest the one 60 feet hig}-put each end and 100 feet long is the fest, although you had better shorten the flat top, otherwise you may have selectiviiy trouble. 100 feet total length in*inding the lead in is abont all that is sequire:| for a modern eight valye set. Nevyertheless try the big aerial by erecting it on halyards so that it may be let down, and if it should cause the set to be unselective. you can let it down and put on longer spacing wires and so shorten it. 6625 -t wish’ to use